As electronic printed circuit assemblies (PCA""s) become more complex, it becomes increasingly difficult for traditional methods to verify their performance and to discover which part or parts are defective. This difficulty has led to extensive automated testing of PCA""s, among the strategies of which are tests to verify the integrity of the solder joints and compression joints that electrically connect (and often mechanically attach) the various components to the PCA, including integrated circuits (IC""s). This is particularly important when dealing with large surface mount components having ball grid arrays (BGA""s), both at the time of initial manufacture and subsequent to a repair. Such BGA parts might be soldered to a matching land grid array (LGA), or held in place by a back-up plate that also squeezes the parts against the LGA through an intervening fuzzy wire ball BGA xe2x80x9csocket.xe2x80x9d Testing is also important for IC""s that have other styles of attachment such as xe2x80x9cJxe2x80x9d lead and a gull wing for surface mount applications, and rod-like or bar-like pins that extend from the package and whose distal ends are either soldered into through holes in the PCA or are pressed into the recess of a socket.
In the present disclosure, the term xe2x80x9cpinxe2x80x9d is used generically to refer to any exposed IC terminal that is connected to a PCA through any of the methods described herein. Accordingly, this term is intended to encompass xe2x80x9cpins,xe2x80x9d the rods or tapered straps predominantly associated with earlier package styles, as well as terminals configured as bumps or lands. The term xe2x80x9cexternal continuityxe2x80x9d is used herein to describe electrical conductivity between an IC pin and the PCA.
Several methods are currently used to verify the external continuity of an electrical connection between a pad or land on a PCA and a pin on an IC. In one such method, the PCA is first mounted in a test fixture which registers it in a known manner upon a test apparatus. A bed of nails or a moveable test probe is brought into contact with traces on the PCA. A computer then controls testing of the individual connections on the PCA by, for example, closing switches of a matrix of relays associated with the bed of nails, or moving the test probe from point to point. During this testing, stimulus is applied to the PCA and measurements are taken. Unfortunately, this procedure tends to provide an unreliable indication of actual continuity between the IC pin of interest and the point of stimulus due to the existence of other paths in the surrounding circuitry.
In another method, electric fields emitted by the PCA are sensed with a capacitive probe whose location is also controlled to match the test being performed. Although capacitive probes often function satisfactorily, they can be rendered useless by internal shielding that may be incorporated into the IC by its manufacturer.
In yet another method, a magnetic field created by current flowing through the pin of interest is sensed. Such measurement of a magnetic field is usually possible, even while ignoring the particular functional circuitry contained in the IC to cooperate with that pin, since there are usually protection diodes connected from each pin to GND and/or to VCC. Although a DC signal can be used as a stimulus, AC signals are often preferred, since: (a) they are easily sensed, often simply with a coil of wire inductively coupled to the resulting magnetic field; (b) they are easier to identify or distinguish a signal of known frequency and phase in an electrically noisy environment; and (c) the magnetic field for low level DC signals can be obscured by external, environmental, or the earth""s magnetic fields. In any event, external biasing of the IC""s protection diodes or simply accepting rectification produced by those diodes will permit a steady or pulsating current to flow, either of which produces a magnetic field detectable by an appropriate sensor positioned proximate the IC. This kind of a system has the additional advantage of testing internal continuity as part of the test for external continuity.
A pulsating system of the sort described above is disclosed in U.S. Pat. No. 5,399,975 issued to Laing, et al. The system described therein can however render inaccurate readings due to conductors on the PCA connected to the pin being probed. In particular, current flowing in the conductor can produce a collateral magnetic field that masks an absence of the magnetic field from the IC, resulting in an undetected open circuit. Alternatively, a false failure indication can occur which can result in indictment of a fully operable component.
In a system described in U.S. Pat. No. 5,631,572 issued to Sheen et al., a spiral antenna is positioned above the IC to induce a current in a conductive closed loop path formed by probes that probe the IC, the protection diodes inside the IC, and the external current measurement circuitry connecting the probes. The antenna is positioned in close proximity to the IC to ensure good coupling. When the spiral antenna is centered over a conductor of the IC, the current flowing through the conductor can induce a current, in accordance with Lenz""s law, that flows in an opposite, parallel direction. This opposing flow causes the currents in the conductor to sum to zero. This phenomenon creates a false open which can skew test results.
From the foregoing discussion it can be understood that there is a need for an improved method of magnetic sensing to provide a better complement to the capacitive method. In particular, it would be desirable to have a magnetic method for checking both internal and external continuity whose reliability is largely unaffected by conductors near the part under test, regardless of their orientation, and that produces a meaningful and usable indication despite influence from other magnetic fields.
The present disclosure relates to a method for determining the electrical continuity for an element of an electrical component, for example, a pin of a printed circuit assembly. The method comprises the steps of supplying an electrical stimulus to the element of the electrical component, positioning a sensor adjacent the element of the electrical component, the sensor having multiple axes along which the sensor is responsive to magnetic fields, receiving magnetic field signals created by the element of the electrical component with the sensor, producing electrical signals indicative of the magnetic field strength sensed by the sensor in multiple directions that correspond to the multiple axes, and comparing the electrical signals with predetermined limits associated with the element being tested. In a preferred arrangement, the sensor is provided with three axes which are oriented in orthogonal directions such that the magnetic signals from the element can be detected in three dimensional space.
The present disclosure also relates to an apparatus for testing the continuity between an element of an electrical component. The apparatus comprises a stimulus system electrically adapted to be coupled to the element of the electrical component, a first sensor oriented along a first axis and inductively coupled to the stimulus system, a second sensor oriented along a second axis and inductively coupled to the stimulus system, a first signal measurement circuit coupled to the first sensor, a second signal measurement circuit coupled to the second sensor, and a comparison circuit coupled to the first and second signal measurement circuits which indicates whether signals induced in the first and second sensors are within predetermined limits. In a preferred arrangement, the apparatus is provided with three sensors which are oriented in orthogonal directions such that the magnetic signals from the element can be detected with the apparatus in three dimensional space.
The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.